1. Field of the Invention
The invention in general relates to voltage probes for passing a test signal from an electronic circuit element to be tested to an oscilloscope or other electronic measurement device, and more particularly to such a probe that has multiple leads densely packed into a small area.
2. Description of the Related Art
Voltage probes are commonly used to pass analog test signals from a circuit under test to an oscilloscope or other electrical or electronic test instrument. Such an electronic probe must be capable of passing an electrical signal on a node or pin of the circuit under test to the test instrument without distorting it, i.e. with high signal integrity. Further, it should not apply any voltage or current to the circuit under test. Present-day electronic circuits operate over frequencies from DC to several gigahertz. Thus, test probes capable of being used with a wide variety of circuits must be able to provide high signal integrity over a wide band width of frequencies.
Integrated and hybrid circuits are becoming both more complex and smaller, leading to ever higher numbers of package leads crowded into less and less space, that is, the leads are becoming extremely dense with very tight pitches. The art has responded to this requirement by producing large numbers of gadgets designed to interface with circuit packages, such as plastic quad flat packs (PQFP). These gadgets provide an interconnect between the dense array of output pins on a circuit package to a less dense array of outputs that can be more easily contacted manually with a hand held probe.
It is time consuming to measure signals one at a time by hand. Thus it would be useful to have a probe system having a high density of probe channels that could be electronically selected via a computer or other selection means. Such a probe system has been provided in our copending United States patent application Ser. No. 08/369,607, pending. However, in this probe system, the high density of wiring in the circuit under test and the interconnects is continued through the probe itself. That is, the probe has essentially the same high density of circuitry as state-of-the-art integrated and hybrid circuits. The closeness of the individual leads in such high density circuits generally results in coupling between the leads and associated noise, distortion, etc.
Up to now, most, if not all, high density packages containing integrated circuits have been digital circuits. In the digital circuit art, high signal integrity is not a significant goal, since digital circuits only need to detect the rise or fall of a digital signal. Thus, in digital circuits, there is a much larger degree of tolerance of coupling, noise, distortion, etc. than there can be in analog instrumentation circuits. Moreover, digital circuits have no need to operate without distortion over a wide band width of frequencies, since most are in digital circuits minor waveform variations are unimportant.
Thus, the development of a probe system with a high density of probe channels that could be electronically selected did not seem possible, since such a system appeared to be inconsistent with the high signal integrity, high band width requirements of present-day probe systems.